The packaging industry, particularly the food packaging area thereof, is currently utilizing large quantities of flexible film laminates. Since all the properties desired in such laminates are not available in any one specific film, the industry generally employs laminates prepared from a combination of films. Very often these laminates are formed from a film of polyethylene terephthalate (PET), polyamide or cellophane, either uncoated or coated with polyvinylidene chloride (PVDC), laminated to a heat sealable polyolefin film which has been treated by corona discharge for adhesion promotion.
In forming such laminates, an adhesive is first coated onto one of the film surfaces. The coated film is then heated to remove any volatiles from the adhesive and thereafter heat combined between laminating rolls to the other film. In view of the process requirements wherein it is not desirable to employ an adhesive which cures in the oven or during heat combining, it is necessary to employ an adhesive which will cure at room temperature after the laminate has been formed. Moreover, the resulting laminate must develop a strong cured bond which is resistant to inherent delaminating stresses due to differences in modulus of the laminae as well as to the added stresses imposed in end-use service on exposure to heat, cold, humidity, etc.
Heretofore, the most satisfactory laminates, as indicated by industry acceptance, have been formed using urethane or polyester-based adhesives. Most of these adhesives, however, have the disadvantage of requiring organic solvents such as methyl ethyl ketone, ethyl acetate or alcohol in order to form an applicable solution. Due to the desirability of eliminating solvents from such adhesives because of their increasing cost, flammability as well as pollution considerations, the development of an aqueous emulsion adhesive system capable of performing comparably to the solvented adhesives becomes vital to the continued growth of the industry.